Anodizing process and additive for anodizing aluminum and its alloys



United States Patent Ofitice 3,434,943 Patented Mar. 25, 1969 This invention relates to and has for its object the production of anodized aluminum oxide coatings which are of superior hardness. It is also related to and has for its one object the processes for the production of such coatings.

It is well known that aluminum and its alloys, herein collectively referred to as aluminum, can be electrolytically anodized, employing anodizing electrolytes such as sulfuric acid, sulfamic acid, oxalic acid, chromic acid and phosphoric acid by passing an electric current through an electrolytic cell of which the aluminum is one electrode and wherein an electrolyte is employed. The most common electrolyte employed is sulfuric acid.

I have discovered that aqueous extracts of certain woods, particularly when obtained from quebracho, when added to the anodizing acid electrolytes herein referred to in the above procedure, will produce harder coats than can be obtained from the same electrolytes in like conditions when not employing the extract of my invention.

Red quebracho is loxopterygium lorentii of the family anacardiaceae. The white quebracho is aspidosperma quibracho. The wood or bark of the quebracho tree is first treated with water to extract the active ingredient therein, which is a derivative of orthodihydroxy benzine as near as can be determined, but I do not wish to be held to this formula or chemical element. The extract and the wood chips are separated and the wood is thrown away. Water is permitted to evaporate from the extract either by heating or naturally until the extract is thick similar to a gelatinous mass. This mass (like resin) becomes quite hard and then can be ground when dry to a powder form.

While I do not wish to be bound by any theory of why my additive produces superior results, I have observed that the additive reduces the rate of corrosive attack of the anodizing acid upon the aluminum. I have also observed that the oxide coats produced employing my additive, when mixed with sulfuric acid as the anodizing electrolyte, contain materially less sulfate ion than do coats produced employing the acid without the additive.

I have observed that the extract has a chelating action in that it will sequester metal ions, for example, iron. It is possible, although I do not wish to be bound by this theory, that the property of the additive to produce harder coats may in part arise from chelating action of the additive.

Whatever the explanation, observation does show that I may produce harder coats by using my additive than may be produced by the same process without the additive of my invention.

Corrosion inhibiting test Two identical specimens of 1100 aluminum (pure aluminum) were immersed completely, each in identical glass jars with tight covers, one containing a solution of 16% by volume of 66 B. sulfuric acid in water, and the other the same but with 6.336 grams per liter of dry powdered quebracho extract. The acid without the additive is shown as No. 1 in the following table, and the one with the quebracho as No. 2. These specimens were periodically removed from the jars, washed, dried, and weighed, then returned to their respective jars. The table shows percent loss in weight of each at progressively longer time of exposure.

Exposure hours No. 1, percent N o. 2, percent Numerous tests were run on three popularly used aluminum alloys in a rectangular anodizing tank of five gallon capacity, provided with refrigeration for the electrolyte. The procedure was to fill the tank with a water solution of sulfuric acid containing 16% by volume of 66 B. acid, after anodizing a few specimens using this electrolyte, dry powdered quebracho extract, in the proportion of .792 gram per liter of electrolyte. Thus, in making comparisons, there was no question as to the relative concentration of the sulfuric acid, with and without the additive, as the very small volume of the additive did not change the concentration of the electrolyte to more than an infinitesimal degree. The alloys used were 2024T3, 606l-T6, and 7075-T6. The tank was made of type 316 stainless steel, and as is customary in the art, it served as the cathode.

Current density comparison Test No. l.Aluminum alloy panel, 2024 T3 alloy, electrolyte 16% by volume 66 B. sulfuric acid in water, duration of test 16 minutes, temperature 303l F., thickness of coating .0014", voltage start 20 volts, end 64 volts. Average current density 71.16 amps. per sq. ft. Power consumed 86.52 watt hours.

Test No. 2.Identical alloy and dimensions of specimen. Electrolyte same as No. l, but with powdered quebracho extract .263 gram per liter added. Time and coating same, temperature 28-31 F. Voltage increments same, from 20 to 64 volts. Average current density 63 amps per sq. ft. Power consumed 70.30 watt hours.

Thus, lower current density and watt hours from electrolyte No. 2.

It was found that very satisfactory oxide coatings of .002 could be attained with the plain acid in 32 to 35 minutes, but all attempts to reduce this time by running the voltage (and thereby the current density) by increments of much more than one volt per minute, resulted in loss of coating in spots or generally by breaking down the coating and sulfating the specimen, or as it is expressed by those working in the art by burning of the specimen. In many instances the voltage had to be left at one value for 2 or more minutes to obviate burning.

On the contrary, however, when using an electrolyte provided, as mentioned supra, with the quebracho extract, it was found possible to attain .002" coatings in 20 minutes or less by increasing the voltage (and thereby the current density) by increments of more than one volt per minute. The coatings thus produced were found to be superior to those produced by the electrolyte without the quebracho extract. An experienced anodizer can tell that burning has started by observing the ammeter. If the current starts to rise, while the voltmeter remains constant or starts to decrease, burning has started. This is well known to those skilled in the art.

Following is a table showing the results of six of these tests, on panels of the indicated alloys, and 4" x 4" X A" or 4" x 4 x A2", each provided with :a A" hole clear through from the center of one face to that of the opposite face. One face and the four edges were not masked, but a rubber stopper was inserted tightly into the hole, and

one face was masked with anodizers tape extending from the top of the hole to the top of the panel. Thus the panel was subject to anodic oxidation, except for the masked portions which remained bare metal after anodizing. This arrangement is used by many skilled in the art, and adapts the specimen to the use of the well known Taber Abraser, as well as enabling the anodizer to measure the coatingthickness with a micrometer caliper or to adjust the zero of a thickness measuring instrument, such as the Dermitron or the Permascope, both well known to those skilled in the art. The temperature of the electrolyte was at 24 to 26 F. at the beginning of a test, and rose to from 27 F. to 34 F. at the end of a test. The current density (density in the table) is reported in amperes per square foot of exposed area of the specimen and is computed from the average of all current readings on the ammeter, added together, and divided by the number of readings, and this average current divided by the exposed area of the specimen. The racks with which the specimens were held, and through which current was delivered to the specimens were masked with anodizers tape and with wax applied hot, so that only the specimen was exposed to the electrolyte, except for the minute points of contact between rack and specimen. Thus the area of the rack was not exposed to the electrolyte to give false readings.

After being washed well in cold water, until all bubbles of gas had ceased to exude from the oxide surfaces, after removing the specimen from the anodizing tank, the specimen was removed from the rack and wiped dry, after which it was weighed, and placed in an oven maintained at 180 F. for 55 minutes, then cooled in a vacuum chamber, reweighed, and subjected to 10,000 cycles in the Taber Abraser, reweighed, subjected to 30,000 more cycles, reweighed, then the loss in coating thickness measured with a Dermitron. The Taber Abraser was provided with CS-17 abrasive wheels in all the tests. The breakdown voltage of all of these specimens was 1700 volts, except N0. 4 which Was 1800 volts.

Duration Thickness Density N Alloy Electrolyte (min) (in.) (amps per sq. ft.)

35 0020 29. 4 26 0020 40. 46 32 0020 35. 90 e 28 0020 45. 43 Plain 32 .0022 45. 40 With additive- 21 0022 57.07

Wt. loss Wt. loss Thickness No. 10,000 cycles 40,000 cycles ss (gram) (gram) (inch) This additive has been used in concentrations of the following proportions in 16% by volume sulfuric acid: .660 gram per liter, .792, 1.320, 3.168, and 6.336 grams per liter. These have all proven successful; however, for general use, relative cost taken into account, it has been found that .792 gram per liter is highly satisfactory. Besides using 16% acid as described supra, a concentration of 9. 6% by volume of 66 B. sulfuric acid has been used with .792 gram per liter of dry, powdered quebracho extract, and found to be capable of achieving very satisfactory oxide coatings of .002 inch thickness, but at this acid concentration more care must be exercised to avoid burning.

I have tried adding ferrous sulfate in an amount equal to the amount of quebracho, namely, .792 gram per liter of each, and have attained a coating thickness of .002 inch on 2024 alloy in the same time duration that has been used without the iron sulfate. Iron, therefore, seems to be no detriment.

I have prepared an electrolyte 0f 16% sulfuric acid, wherein .792 gram per liter of quebracho extract, and .792 gram per liter of manganous sulfate were added. With this electrolyte I have attained an oxide coating of .0020 inch in 26 minutes on 2024 alloy, and a coating thickness of .0036 inch in 24 minutes on 6061 alloy.

Using .792 gram per liter of dry powdered quebracho extract alone in 16% by volume 66 B. sulfuric acid, on a .040" x 3 x 5" panel of 2024T3 alloy I have attained a coating thickness of .006 inch in 147 minutes at an average current density of 32.4 amperes per sq. ft.; and with the same electrolyte, I have attained a coating thickness on 6061-T6 alloy of .0028 inch in 24 minutes at an average current density of 46.2 amperes per sq. ft., the panel being x 1" x 6"; and on a similar panel a coating thickness of .0058 inch in 93 minutes with an average current density of 32 amperes per sq. ft.

Thus it becomes evident that my quebracho additive with or without iron or manganese is beneficial for thin and for relatively thick hard anodic coatings on aluminum alloys as it allows a higher current density with less risk of burning, and thereby accomplishing the same thickness in less time; and using the same time interval for the same thickness of coating it uses a lower current density; and also the coatings are harder and more abrasion resistant than those accomplished with 16% acid without additive.

While my additive has been demonstrated as described supra for hard anodizing, I have found that it is also useful for soft and color anodizing. At a tank temperature of F. to F., and using 16% acid with .792 gram per liter of dry powdered quebracho extract, I have anodized specimens, followed by a cold water rinse, and then a subsequent treatment at F. to F. for 10 to 20 minutes in water containing a suitable amount of appropriate organic dye. I have obtained excellent colored coatings in this manner. Thus, there is no need to change the electrolyte, but only the temperature, to switch from hard anodizing to soft and color anodizing, and vice versa.

The additive of my invention may be made by extracting the quebracho wood, as described in my patent referred to above, No. 2,838,877. Wood or bark in the form of chips or sawdust, may be steeped in water at temperatures from room temperature to temperatures above 212 F. using pressure cookers. The lower the temperature, the longer it takes to perform the extraction and attain the low pH produced by the extracted acid. The pH may be in the region of about 2 to about 5 depending on the time taken in the extraction and the temperature. Preferably, I steep sawdust or chips of the quebracho, herein collectively referred to as quebracho, in water heated to a temperature in the range of about 180 F. to boiling at atmospheric pressure, i.e., 212 F. or heat to a more elevated temperature under superatmospheric pressure, and filter off the sawdust or chips. The additive may be added to the acid to produce the desired concentration of additive. About 1% to 30% by volume 66 B. sulfuric acid to the mixed electrolyte may be employed. Preferably, I employ from about 5 to 20 parts of volume of 66 Baum acid to make up 100 parts by volume of the mixed water and additive.

To this dilute sulfuric acid I add a small portion of the above extract, employing from about 2% to 29% by volume based on the mixed acid-water and additive.

The anodizing procedure employing the mixed electrolyte may be any of the processes of the prior art, such as are employed in producing hard coating of the prior art. The voltage, current densities and rate of rise of voltage such as are employed in well known prior art processes for anodizing aluminum may be employed. The temperature employed is preferably in the range of above freezing to about 50 F. but preferably a temperature of about 35 F. is desirable. Temperatures in the range of 10 to 15 F. are also suitable. Agitation to produce uniform coats is preferably employed.

The conditions and procedures conventionally employed in anodizing employing sulfuric acid electrolyes may be followed. Thus, the voltage may be raised at the rate of 1 volt per minute for the time necessary to deposit the coat desired. Alternatively, a constant current density may be maintained by controlling the voltage application as the resistance of the cell increases, due to deposit of aluminum oxide on the electrode.

The current densities which may be employed will vary over a wide range, depending upon the geometry of the system, the internal cell resistance and the total number of square feet of surface exposed for anodizing. United States Patent No. 2,692,851, issued to Charles F. Burrows, is referred to for a description of a sulfuric acid anodizing process and the conditions for operation, as illustrated in the prior art procedures. The procedures described in said patent and other of those conventionally previously employed in the prior art may be employed in operating in the processes of my invention.

During the anodizing process it is of importance to agitate the electrolyte, and to also introduce a continuous supply of air or oxygen thereto. This is accomplished by means of an air pump or compressor, the outlet of which extends into the tank of electrolyte and bubbles air through the electrolyte and around the object which is being anodized. Thus a continuous and ample supply of oxygen is provided for the chemical oxidation reaction within the tank. This step in the process decreases the time required to anodize a sheet of aluminum, and also provides a more effective coating therefor.

Having described my invention, I claim:

1. The process of anodizing aluminum and its alloys comprising subjecting said aluminum anode to electrolytic action in an electrolytic cell of which the electrolyte is a water solution comprising a mixture of anodizing acid and an aqueous extract of quebracho.

2. The process of claim 1, in which said electrolyte is obtained by mixing an extract of quebracho having a pH of from about 2 to about 5, sulfuric acid and water.

3. The process of claim 1, in which the said extract is from about .1 to about 15.0 grams per liter of the electrolyte mixture.

4. The process of claim 1, in which the sulfuric acid is 66 Baum acid from about .1 to about 15.0 grams per liter of the electrolyte mixture.

5. The process of claim 1 and in which the aqueous extract is obtained by steeping quebracho in water at a temperature within the range of ambient temperature up to the boiling temperature, for a time sufficient to produce an extract having a pH of from about 2 to about 5.

6. The process of claim 1, in which the electrolyte is sulfuric acid.

7. An aqueous anodizing electrolyte as specified in claim 1, and wherein the electrolyte consists essentially of a mixture of water, sulfuric acid, and an aqueous quebracho extract, said extract having a pH of from about 2 to about 5.

8. The aqueous electrolyte recited in claim 7, and wherein the extract is from about .1 to about 15.0 grams per liter of the electrolyte.

9. The process of anodizing aluminum and its alloys comprising subjecting said aluminum anode to electrolytic action in an electrolytic cell of which the electrolyte is a water solution comprising a mixture of the anodizing acid and an aqueous extract of quebracho with the addition of manganese ion.

References Cited UNITED STATES PATENTS 6/1958 Working 47-58 6/1968 Working 204-58 

1. THE PROCESS OF ANODIZING ALUMINUM AND ITS ALLOYS COMPRISING SUBJECTING SAID ALUMINUM ANODE TO ELECTROLYTIC ACTION IN AN ELECTROLYTIC CELL OF WHICH THE ELECTROLYTE IS A WATER SOLUTION COMPRISING A MIXTURE OF ANODIZING ACID AND AN AQUEOUS EXTRACT OF QUEBRACHO. 